First constraints on the local ionization topology in front of two quasars at z ~ 7.5

Timo Kist, Joseph F. Hennawi, Frederick B. Davies, Eduardo Bañados, Sarah E. I. Bosman, Zheng Cai, Anna-Christina Eilers, Xiaohui Fan, Zoltán Haiman, Hyunsung D. Jun, Yichen Liu, Jinyi Yang, Feige Wang

First listed 2025-08-29 | Last updated 2025-08-29

Abstract

Thus far, Lyman-$α$ damping wings towards quasars have been used to probe the \textit{global} ionization state of the foreground intergalactic medium (IGM). A new parameterization has demonstrated that the damping wing signature also carries \textit{local} information about the distribution of neutral hydrogen (HI) in front of the quasar before it started shining. Leveraging a recently introduced Bayesian \texttt{JAX}-based Hamiltonian Monte Carlo (HMC) inference framework, we derive constraints on the Lorentzian-weighted HI column density $N_\mathrm{HI}^\mathrm{DW}$, the quasar's distance $r_\mathrm{patch}$ to the first neutral patch and its lifetime $t_\mathrm{Q}$ based on JWST/NIRSpec spectra of the two $z \sim 7.5$ quasars J1007+2115 and J1342+0928. After folding in model-dependent topology information, we find that J1007+2115 (and J1342+0928) is most likely to reside in a $\langle x_\mathrm{HI} \rangle = 0.32_{-0.20}^{+0.22}$ ($0.58_{-0.23}^{+0.23}$) neutral IGM while shining for a remarkably short lifetime of $\log_{10} t_\mathrm{Q} /\mathrm{yr} = 4.14_{-0.18}^{+0.74}$ (an intermediate lifetime of $5.64_{-0.43}^{+0.25}$) along a sightline with $\log_{10} N_\mathrm{HI}^\mathrm{DW} /\mathrm{cm}^{-2} = 19.70_{-0.86}^{+0.35}$ ($20.24_{-0.22}^{+0.25}$) and $r_\mathrm{patch} = 28.9_{-14.4}^{+54.0} \,\mathrm{cMpc}$ ($10.9_{-5.9}^{+5.6} \,\mathrm{cMpc}$). In light of the potential presence of local absorbers in the foreground of J1342+0928 as has been recently suggested, we also demonstrate how the Lorentzian-weighted column density $N_\mathrm{HI}^\mathrm{DW}$ provides a natural means for quantifying their contribution to the observed damping wing signal.

Short digest

Uses JWST/NIRSpec damping-wing fits and a three-parameter local model to extract the Lorentzian-weighted HI column N_HI^DW, the distance to the first neutral patch r_patch, and the quasar lifetime t_Q in front of two z ≈ 7.5 quasars. J1007+2115 favors a moderately ionized IGM with ⟨x_HI⟩ = 0.32^{+0.22}_{-0.20}, an exceptionally short lifetime log10 t_Q/yr = 4.14^{+0.74}_{-0.18}, r_patch ≈ 28.9 cMpc, and log10 N_HI^DW ≈ 19.70, while J1342+0928 implies a higher neutral fraction ⟨x_HI⟩ = 0.58^{+0.23}_{-0.23}, an intermediate lifetime log10 t_Q/yr = 5.64^{+0.25}_{-0.43}, r_patch ≈ 10.9 cMpc, and log10 N_HI^DW ≈ 20.24. Mapping these local constraints through topology-informed priors links the sightline-level topology to global reionization timing. The N_HI^DW metric also naturally captures potential contributions from foreground absorbers toward J1342+0928.

Key figures to inspect

• Figure 1: Compare local vs global fits to J1007+2115 around the Lyα red wing; the amplitude/shape that the local model attributes to N_HI^DW and r_patch is visible in the separation between the absorbed model and reconstructed continuum, with uncertainty bands showing how tightly the red wing is pinned.
• Figure 2: Inspect the posteriors for J1007+2115—note the tight preference for a very short t_Q (~10^4.1 yr), large r_patch (~29 cMpc), and low N_HI^DW, and how the topology-informed mapping yields ⟨x_HI⟩ ≈ 0.32 consistent with the direct global fit.
• Figure 3: For J1342+0928, the stronger damping wing drives higher N_HI^DW and a nearer first neutral patch (r_patch ~11 cMpc); contrast the inferred continuum and residuals with J1007+2115 to see why the lifetime shifts to an intermediate value.
• Figure 4: Posterior comparisons for J1342+0928 including overlays from putative local absorbers; use the N_HI^DW axis to gauge how much of the damping wing could be explained by CGM/IGM absorbers under different metallicity assumptions.